Bump protobuf version to beta-3.3

It’s just the merge of beta-3.1 and beta-3.2, both of which we need.
diff --git a/examples/cpp/helloworld/Makefile b/examples/cpp/helloworld/Makefile
index 780e5e4..b45b3c7 100644
--- a/examples/cpp/helloworld/Makefile
+++ b/examples/cpp/helloworld/Makefile
@@ -97,7 +97,7 @@
 	@echo "Please install Google protocol buffers 3.0.0 and its compiler."
 	@echo "You can find it here:"
 	@echo
-	@echo "   https://github.com/google/protobuf/releases/tag/v3.0.0-beta-2"
+	@echo "   https://github.com/google/protobuf/releases/tag/v3.0.0-beta-3.3"
 	@echo
 	@echo "Here is what I get when trying to evaluate your version of protoc:"
 	@echo